The present invention generally relates to speaker enclosures, and more specifically to a symmetrical air friction enclosure for speakers which comprises an improved labyrinth type enclosure which provides good performance in a small external volume.
Labyrinth-type speakers are well known. The first speaker enclosure that did not increase the speaker free air resonance was patented in 1936 by Benjamin Olney (U.S. Pat. No. 2,031,500). The typical acoustic labyrinth is essentially a long folded tube, acoustically lined with acoustical material, mounted behind the speaker. The acoustical material is used in the labyrinth to increase the effective length of the tube as seen by the backwave. However, such stuffing or lining has a tendency to over-damp the low frequencies being produced.
The effective length of the labyrinth tube is generally selected to be approximately equal to one-half the wavelength of the free air resonance frequency of the loudspeaker. At this length, the backwave is essentially shifted 180.degree. out of phase and reinforces the frontwave. At one-quarter the wavelength, the anti-resonant action of the pipe offers maximum damping to the speaker. On the other hand, as mentioned above, at the half wavelength frequency, the emerging wave is in phase with that coming from the front of the cone, adding to the speaker's output. A discussion of labyrinth speakers is presented in "Popular Electronics", January, 1972, at page 40.
Since the length of the tube of a labyrinth speaker is generally related to the wavelength at the free air resonance frequence of the loudspeaker, and such free air resonance frequencies can be as low as 30 Hz., the effective lengths of labyrinth tubes can be greater than 7 feet long. Accordingly, labyrinth speakers are generally large in size, bulky and have large volumes.
The fold in the tube of the labyrinth is generally accomplished by use of one or two partitions within the enclosure. However, this leaves substantial portions of the cabinet or enclosure unbraced and therefore susceptible to undesired vibrations and induced resonances.
Other types of non-labyrinth speakers are known which utilize various partitions therein to either increase the effective length of a path, to produce a horn having an increasingly large cross-sectional area, or to serve as baffles. For example, U.S. Pat. No. 3,327,808 discloses a loudspeaker housing having as its object to obtain an improved bass response. However, the loudspeaker housing disclosed in this patent is for a resonant column-type enclosure, which consists of a speaker at one end and a series of constrictions at the other end of a tube. The constrictions appear in the housing outer wall and the constructions or apertures have dimensions selected to make the same frequency selective by producing inductive effects and acting to cut off higher frequencies. The resonant column may be folded or telescoped but it is shorter in length than a labyrinth. More importantly, the resonant column increases the free air resonance frequency of the speaker housed in it, and has an irregular polar sound distribution curve. This is its main disadvantage. For a discussion of resonant column-type enclosures, see "How to Build Speaker Enclosures", by Alex Badmaieff and Don Davis, Howard W. Sams and Co., 1973, page 115. In introducing the invention, the patentee of this patent distinguishes his invention by stating that his structure should not be confused with an acoustic labyrinth.
In U.S. Pat. No. 2,646,852, for a loud-speaker cabinet, the patentee discloses an enclosure provided with a plurality of internally spaced partitions. The partitions and apertures or openings between adjacent chambers or compartments is selected to provide one tortuous path which leads to a closed end, the reflected sound being retransmitted through its initial path and ultimately out through an output port located proximate to the loudspeaker. This construction is not a labyrinth type of construction and tends to increase the free air resonance frequency of the speaker.
Numerous constructions are also known which utilize internal partitions to generate or form a tortuous path for the front or backwave produced by a loudspeaker. For example, the following U.S. patents disclose various constructions of generally horn-type enclosures: U.S. Pat. Nos. 2,224,919; 2,310,243; 2,971,598; and 3,642,091. In each case, the partitions are generally arranged to produce a tortuous path which has an increasingly large cross-sectional area to either the front or to the backwave. Accordingly, the devices disclosed in these patents are not true labyrinth speakers and do not have the desirable characteristics thereof.
A twin equilateral sound speaker enclosure is disclosed in U.S. Pat. No. 3,529,691. The primary object of this device is to provide 360.degree. dispersion of the sound over a wide frequency range. This is achieved by utilizing an enclosure provided with three substantially concentric tubes together forming a continuous path. However, the loudspeaker is mounted in the central portion of the enclosure in such a manner that it is the front wave which is caused to propagate through the tortuous path formed by the tubes while the back of the speaker is completely enclosed.
In U.S. Pat. No. 1,810,708, a method and apparatus for amplifying sound waves is disclosed. While the device disclosed utilizes a plurality of generally concentric cylindrical walls, this device is not an enclosure for a speaker. Additionally, since the cylindrical partitions each form a substantially annular space or chamber, reflections of the wave at the axial ends of the chambers from one chamber to the next in effect produces a horn-type enclosure where the cross-sectional area for the wave front increases step-wise instead of gradually and continuously as with conventional horns.